ARTICLE pubs.acs.org/EF
Characterization of the Diesel Soot Oxidation Process through an Optimized Thermogravimetric Method J. Rodríguez-Fernandez,* F. Oliva, and R. A. Vazquez Escuela Tecnica Superior de Ingenieros Industriales, University of CastillaLa Mancha, Edificio Politecnico, Avenida Camilo Jose Cela, s/n, 13071 Ciudad Real, Spain ABSTRACT: A thermogravimetric analyzer (TGA) was used to oxidize real diesel soot produced in a modern four-cylinder, turbocharged, diesel engine equipped with a common rail injection system. In the first part of the study, a large amount of soot produced with a variety of diesel fuels of current and future interest and in typical engine modes was collected. A thorough test matrix was designed and executed in the TGA to develop an optimized oxidation method, which allows for the determination of oxidation profiles, characteristic temperatures, and Arrhenius kinetic parameters. Such parameters are critical for modeling diesel filter regeneration and designing more efficient regeneration techniques. A comparison of this method to others proposed in previous literature is made, and the advantages are pointed out. In the second part, diesel and biodiesel soot produced in three engine operating modes (low, medium, and high load) was evaluated with the proposed method. The results showed that the effect of the fuel on the oxidation process was more significant than that of the engine mode. Biodiesel soot oxidation occurred at lower temperatures, proving the possibilities that this fuel offers for achieving more efficient filter regeneration. Although differences in the activation energy of diesel and biodiesel soot were not large, the oxidation rates of biodiesel soot were, on average, 1 order of magnitude above those of diesel soot.
1. INTRODUCTION For the last few decades, a growing concern has been developed in the transport sector regarding the rise of emissions, which contribute to global warming and are harmful to the environment and human health.13 A reduction of these emissions has been encouraged from local governments and other sectors involved.4,5 In the past, the emission reduction was possible through the promotion of more efficient diesel vehicles and the application of techniques capable of reducing nitrogen oxides (NOx) and particulate matter (PM) emissions, such as the exhaust gas recirculation (EGR) or the adjustment of injection (timing, pressure, and strategies).69 More recently, the use of alternative fuels1013 has been promoted as well. However, in the present and future, those techniques are no longer sufficient to meet the more and more stringent emission regulations. In Europe, the new Euro 5/Euro V standard imposed a drastic reduction in PM (for cars) and NOx (for heavy-duty trucks) emissions.14 Most manufacturers had to implement advanced after-treatment devices in their vehicles.15,16 In the case of PM (mainly composed of carbonaceous agglomerates known as soot and adsorbed hydrocarbons on its surface), the solution adopted was the diesel particulate filter (DPF), which collects soot with high filtering efficiencies17 but needs to be periodically regenerated (i.e., soot has to be oxidized to carbon dioxide). Different regeneration techniques have been studied,1719 aiming to balance the energy and time consumption and temperature required. To increase the complexity, soot oxidation in a diesel particle filter is affected by many factors, such as (i) the composition, flow rate, and temperature of the exhaust gas, (ii) the physical, chemical, and structural properties of soot, and (iii) the trap characteristics (shape, material, type, and concentration r 2011 American Chemical Society
of catalysts). Moreover, the heat released during the trap regeneration affects its durability, with the filter thermal runaway being a serious concern for manufacturers.17,18 The determination of the kinetic parameters associated with the diesel soot combustion is critical for traps modeling and will help manufacturers design more efficient regeneration techniques and thermal management strategies.2023 The reactivity of soot toward oxidant agents (O2 and NO2) is a main issue in facilitating the trap regeneration. The higher its reactivity, the lower the temperature needed for oxidation, deriving in economic and energetic benefits. Different analytical techniques have been applied to study the ex situ diesel soot reactivity and its oxidation behavior, such as the thermogravimetric analyzer (TGA), single or combined with differential scanning calorimetry (DSC),2325 micro-Raman spectroscopy and wide-angle X-ray diffraction (XRD) measurements,2628 diffuse reflectance infrared Fourier transform (DRIFT),29 or temperature-programmed oxidation (TPO) reactors.30 Among them, TGA is one of the more widely used because of its simple, versatile performance. In the TGA, the soot sample is subjected to oxidation inside a furnace at a fixed or increasing temperature, while its weight loss is continuously recorded. This single test can be used for determining both the characteristic oxidation temperatures and the kinetic parameters. When diesel soot is studied with TGA, both isothermal and non-isothermal tests are proposed and compared in the literature.31,32 Dependent upon the thermal inertia of the furnace, reproducing a temperature profile may be a problem in Received: February 4, 2011 Revised: April 1, 2011 Published: April 19, 2011 2039
dx.doi.org/10.1021/ef200194m | Energy Fuels 2011, 25, 2039–2048
Energy & Fuels non-isothermal tests. On the contrary, extracting the kinetic parameters of the soot oxidation reaction requires just a single test. Several tests are required for the same task in case isothermal tests are applied.32 Moreover, non-isothermal tests can sweep the whole temperature window where the reaction takes place, offering complete information about the characteristic temperatures associated with the oxidation. Considering the advantages, an optimized non-isothermal method is proposed in the present work. However, two shortcomings are related to the use of the TGA for characterization of the soot oxidation reaction.23,3335 The first one is the apparent dependency of the kinetic parameters upon the experimental conditions selected in the instrument (such as the gas flow rate, the heating ramp, or the initial sample mass). The second limitation is that, under certain operating conditions, the soot oxidation inside the TGA furnace could be a partially diffusion-controlled reaction. In such a case, the kinetic parameters cannot be inferred directly from the TGA curves, and a combination of diffusive and kinetic models is necessary. The present study develops a method to overcome both limitations. After the effect of the experimental conditions is evaluated, an optimized method is proposed, aiming to obtain a good repeatability in the final results, in terms of oxidation temperature and kinetic parameters. Issues such as time consumed to collect the sample and the running time of the method are considered as well. Special care was taken to ensure that the reaction was not controlled by diffusion by avoiding those operating conditions where the oxygen-transfer limitation could be enhanced, such as low gas flow rates, high crucibles, which leave a stagnation region between the top of the crucible and the top surface of the sample, or high initial sample masses. Although some works have been previously published using the TGA with similar purposes, most of them either select non-optimized parameters or do not use real soot generated in diesel engines. Synthetically manufactured soot, such as Printex U and Regal 600,36 charcoal derived from coal pyrolysis,37,38 and/or carbon black39,40 are commonly used instead, but these present characteristics (composition, size, and internal structure) very different from real diesel soot. This method could be easily reproduced in any common thermogravimetric apparatus, thus enabling authors to run their tests under optimized working parameters.
2. EXPERIMENTAL SECTION A four-cylinder, four-stroke, turbo-charged, intercooled, commonrail, 2.0 L Nissan diesel engine (model M1D) with a rated power of 110 kW was used in this study. Its emission reduction system comprises cool EGR (with the use of an internal cooler), a diesel oxidation catalyst (DOC), and a regenerative wall-flow DPF. The main specifications of the engine are given in Table 1. The engine was also equipped with the necessary instrumentation for monitoring and controlling the pressure and temperature in essential locations of the engine. The engine was coupled to an asynchronous electric brake Schenck Dynas III LI 250, which permitted measuring and controlling the engine speed and torque. The software INCA PC and the hardware ETAS ES 591.1 were used for the communication and management of the electronic control unit (ECU). In particular, in the stationary tests, the EGR ratio and injection timing were kept constant by accessing the ECU, because the emissions are strongly dependent upon these parameters. The soot employed for the method optimization (section 4) was different from the soot analyzed in the method validation (section 5). The soot for the method optimization was collected in multiple steady operating modes, extracted from the New European Driving Cycle
ARTICLE
Table 1. Engine Characteristics fuel injection system
Bosch CP1H high-pressure pump electronically controlled (1600 bar) 270 at idle (at 750 min1) and 1323 at
injection pressure (bar)
full load (at 2000 min1) maximum rated power (kW) maximum rated torque (N m)
111 (at 4000 min1) 323.5 (at 2000 min1)
cylinder arrangement
4, in line
bore (mm)
84
stroke (mm)
90
displacement (L)
2.0
compression ratio
16:1
Table 2. Fuel Properties units
diesel (REF)
biodiesel (B100)
density at 15 °C
kg/m3
811
880
distillation T50
°C
238
234
distillation T90
°C
275
335
°C
65 292
63 ND
CFPP
°C
30
3
kinematic viscosity at 40 °C
mm2/s
1.99
4.26
sulfur content
mg/kg
